Influences of cellulose nanofiber on the epoxy network formation

The effects of cellulose nanofiber on the curing behavior, dynamic mechanical, and morphology properties of epoxy/diamine systems were investigated. The studies were conducted using an aliphatic triamine, diethylentriamine (Dien), and an aromatic diamine, diaminodiphenylmethan (DDM), as the curing agents in the presence of three different levels of 0.5, 2, and 5 phr of cellulose nanofiber. Calorimetry experiments were used to probe the changes in the reaction enthalpy and in the glass transition temperature (T_g) as a function of the fiber concentration. The results showed that both the T_g and the heat of reaction were increased with increasing the fiber concentration up to 2 phr. The experimental cure data obtained from in situ FT-IR measurements were used to evaluate the kinetic parameters using the modified Avrami theory of phase change. The results showed that the kinetic parameters were less sensitive to the epoxy composition. DMTA measurements showed that the storage modulus and the T_g of the composites were dependant on the level of fiber loading. SEM studies revealed that a reasonable dispersion and adhesion have been achieved between the fiber and the epoxy matrix at low concentration of fiber. It is concluded that the fiber agglomerated at high concentration of cellulose fiber prevented the formation of a homogeneous mixture, thus resulting in weak thermal and mechanical properties.     

Thermal and dynamic mechanical properties of IPNS formed from unsaturated polyester resin and epoxy polyester

The interpenetrating polymer networks (IPNs) were formed by unsaturated polyester resin (UPR) polymerized by free radical initiators: benzoyl peroxide (BPO) or cumene hydroperoxide (CHP) and epoxy polyester (EP), cured with acid anhydrides: tetrahydrophthalic anhydride (THPA) or maleic anhydride (MA). IPNs consisting 10, 30, 50, 70, 90 wt% of EP were prepared. The effect of the EP component in the IPNs and the type of curing agent on the cure behavior, thermal, and viscoelastic properties have been investigated. The results showed that both EP content and used curing system influenced on studied properties. As the EP content increased, the glass transition temperatures (T _g) also increased. Moreover, higher values of tanδ_max and lower values of cross-linking density in a rubbery state (ν_e) of IPNs containing higher EP content, probably due to plasticization effect of EP component were observed. Additionally, more heterogeneous network structure (higher values of the full-width at half-maximum (FWHM) as the EP content decreased was prepared. The thermal and viscoelastic properties of the blends cured with BPO/MA or CHP/MA system were considerably better than those cured with BPO/THPA or CHP/THPA. The higher stiffness, ν_e, T _g and lower tanδ_max values were obtained. It was probably connected with the interactions of carbon–carbon double bonds of MA with vinyl monomer (styrene), UPR and radical initiators causing to obtain more cross-linked polymer network structure. This supposition was confirmed on basis of the cure reaction monitored by DSC. The chemical interactions between two components of the blends and epoxy hardener caused that the BPO/MA or CHP/MA cure systems influenced on the cure behavior of UPR and EP components in the IPNs. The exotherm peak temperature (T _max1) shifted to lower values compared to these in the neat UPR whilst T _max2 shifted to higher values than in the neat EP. However, the cure behavior of the UPR was not greatly affected by the presence of EP component when BPO/THPA or CHP/THPA cure systems were used due to the lack of chemical interactions between the components and their curatives.     

The β-relaxation in epoxy resins; the temperature and time-dependence of cure

Internal friction and creep measurements have been used to reveal the mechanism of cure in epoxy resins cross-linked with diethylene triamine (DETA). The -relaxation is associated with the main glass transition of the undercured resin network. The glass transition temperature (T _g) is about 40° C above the maximum temperature of cure and the curing reaction slows down about 2 h after each increase of the temperature. At 25° C the cure is only about half complete and since in this resin, when fully cross-linked, T _g is at about 140° C, temperatures of 100° C or over are needed to complete cure.     

Thermal stress hysteresis and stress relaxation in an epoxy film

Thermal cycling of an epoxy coating on silicon through the glass transition temperature (T _g) revealed a large stress hysteresis on the first thermal cycle through T _g and a change in the stress–temperature slope at T _g resulting from the change in the epoxy elastic properties due to the glass transition. This stress hysteresis was not observed on subsequent thermal cycles through T _g. However, after the coating was annealed (aged) below T _g (for hours or longer)—during which the stress relaxed exponentially with time—the stress hysteresis returned. The magnitude of stress hysteresis, on cycling through T _g, was found to correlate to the magnitude of long-time relaxation that occurred during annealing at temperatures below T _g.     

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature (T_g) were studied. The increase in T_g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T_g’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T_g data at various cure states was used to determine use temperature limits for the composite repair system.     

Preparation of Microcellular Epoxy Foams through a Limited‐Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

3D cross‐linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited‐foaming method is introduced for the preparation of microcellular epoxy foams (Lim‐foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim‐foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free‐foaming process (Fre‐foams). Surprisingly, however, the T_g of Lim‐foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross‐linking reaction during post‐curing.     

Effect of DOP-based compounds on fire retardancy, thermal stability, and mechanical properties of DGEBA cured with 4,4′-DDS

The structure-property-relationships, thermal stability and flame retardancy of a DGEBA-DDS system containing various organo-phosphorus compounds as flame retardants is investigated. Three non-reactive (DOP-ethyl, DOP-ethylhexyl and DOP-cyanur) and one reactive (DOP-glycidyl) phosphorous compounds are added separately to the epoxy resin and the mixtures are cured with 4,4′-DDS in a substoichiometric ratio. The addition of such DOPO-compounds leads to improved flame retardancy at low phosphorus contents of about 2 wt.% (about 20 wt.% of additive) without significantly affecting other important properties such as fracture toughness (K_lc) and glass transition temperature (T_g) of the matrix. Neither the type nor the amount of additive affects the fracture toughness of cured epoxies up to additive concentrations of between 18 and 24 wt.%. Furthermore, the loss in glass transition temperature of the cured resin can be correlated with the amount and chemical reactivity of the organo-phosphorus additive. The reactive DOP-glycidyl and the non-reactive DOP-cyanur additive are observed to maintain the highest glass transition temperature of the epoxy system mainly due to a higher extent of the cross-linking reaction. The results presented in this study highlight the potential of optimising the flame retardancy and the resulting physical and mechanical properties of epoxy systems for liquid composite moulding applications by varying the chemical structure of the organo-phosphorus compounds.     

Poly(ether ether ketone) with pendent methyl groups as a toughening agent for amine cured DGEBA epoxy resin

A diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was modified with poly(ether ether ketone) with pendent methyl groups (PEEKM). PEEKM was synthesised from methyl hydroquinone and 4,4′-difluorobenzophenone and characterised. Blends of epoxy resin and PEEKM were prepared by melt blending. The blends were transparent in the uncured state and gave single composition dependent T _g. The T _g-composition behaviour of the uncured blends has been studied using Gordon–Taylor, Kelley–Bueche and Fox equations. The scanning electron micrographs of extracted fracture surfaces revealed that reaction induced phase separation occurred in the blends. Cocontinuous morphology was obtained in blends containing 15 phr PEEKM. Two glass transition peaks corresponding to epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum of the blends. The crosslink density of the blends calculated from dynamic mechanical analysis was less than that of unmodified epoxy resin. The tensile strength, flexural strength and modulus were comparable to that of the unmodified epoxy resin. It was found from fracture toughness measurements that PEEKM is an effective toughener for DDS cured epoxy resin. Fifteen phr PEEKM having cocontinuous morphology exhibited maximum increase in fracture toughness. The increase in fracture toughness was due to crack path deflection, crack pinning, crack bridging by dispersed PEEKM and local plastic deformation of the matrix. The exceptional increase in fracture toughness of 15 phr blend was attributed to the cocontinuous morphology of the blend. Finally it was observed that the thermal stability of epoxy resin was not affected by the addition of PEEKM.     

Curing characteristics of an epoxy resin in the presence of ball-milled graphite particles

The effects of simply-made graphite particles (GPs, 1 wt%) on curing kinetics of an epoxy resin were investigated by means of differential scanning calorimetry (DSC). Two approaches based on constant and variable activation energy were applied to analyze the DSC curves. Results from the constant energy method showed that addition of the GPs increased the activation energy E _c and the overall order of reaction m + n. With the variable energy method, the activation energy varied with curing conversion substantially in the GP/epoxy system; in contrast, the variation in the pure epoxy was very limited. The GPs also decreased the heat of reaction (ΔH) and increased the glass transition temperature (T _g) for the epoxy. Comprehensive analyses indicated that the GPs did not significantly impede the curing reaction, which can enable improvement of the composite functionalities without creating processing penalties. The information obtained from this study provides an understanding of multiple factors involved in the complex relationship of structure and properties of the composites.     

On depression of glass transition temperature of epoxy nanocomposites

Epoxy composite materials filled with nano-alumina particles were prepared by mechanical mixing techniques. The glass transition temperatures (T _g) of the nanocomposites were found to decline significantly with the increasing filler content. After the addition of 30-phr nanoparticles, the T _g of the filled sample decreased by as high as 55 °C, as compared with that of the neat epoxy polymer. Based on the selective adsorption hypothesis and the molecular diffusion, it is speculated that the hardener molecules were unevenly distributed in the nanocomposites, which caused imbalanced stoichiometry between the epoxy and the hardener and finally decreased the T _g. Some results that may support the adsorption hypothesis were given and discussed.     

Isothermal cure characterization of fumed silica/epoxy nanocomposites: The glass transition temperature and conversion

The glass transition temperature (T_g) and conversion (α) were measured for a cycloaliphatic epoxy/anhydride system incorporated with different contents of hydrophobic fumed silica. Samples isothermally cured at varying cure temperatures and times were analyzed by differential scanning calorimetry (DSC). T_g vs. ln(time) data at an arbitrary reference temperature were superposed by time–temperature shifts to form a master curve for the kinetically controlled reaction, and the shift factors were used to calculate an Arrhenius activation energy. Influence of the hydrophobic fumed silica was investigated from T_g and α data. For low conversions, the incorporated fumed silica may reduce segmental mobility, giving rise to an increase in T_g. For high conversions, there was a significant depression of T_g which may be associated with the presence of residual hydroxyl groups on the surface of hydrophobic fumed silica. The positive and negative effects of the fumed silica on T_g and α are discussed.     

Influence of post-curing and temperature effects on bulk density,glass transition and stress-strain behaviour of imidazole-cured epoxy network

The effects of post-curing and temperature on the glass transition, bulk density and stress-strain behaviour in the glassy and rubbery state of 2-ethyl-4-methyl imidazole-cured epoxy network have been evaluated by differential scanning calorimetry (DSC), water displacement and tensile testing. The glass transition temperature,T _g, was found to increase with increasing post-cure temperature and the size of the base line shift in the glass transition region on the DSC thermogram can serve as an indicator of the extent of cure. At room temperature, the decrease in bulk density with increasing extent of cure may be attributed to the additional cross-linking, adding molecular constraints to the thermal constraints. Thus, a higher free volume atT _g can be expected to remain in the glassy state as the sample is slowly cooled through the glass transition temperature. In the investigation on the temperature dependence of the tensile mechanical properties, a fracture envelope was obtained. The tensile strength, Young's modulus and ultimate elongation in the glassy and rubbery state are discussed in detail.     

Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K _Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T _g. It was found that K _Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K _Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K _Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T _g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.     

Synthesis and mechanical properties of cardanol-formaldehyde (CF) resins and CF-poly(methylmethacrylate) semi-interpenetrating polymer networks

Cardanol-formaldehyde (CF) resins (both novolac and resol) and CF-poly(methylmethacrylate) (PMMA) semi-interpenetrating polymer networks were synthesized and their mechanical properties and thermal transitions were evaluated. The lower tensile strength of CF resins compared to phenol-formaldehyde (PF) resins may be understood on the basis of the structure of the C₁₅ side chain imparting steric hindrance and reduction in intermolecular interactions. Interpenetration of CF with PMMA increased the mechanical properties only marginally. Scanning electron micrographs of the semi-IPNs showed two distinct phases. Thermomechanical analysis gave two glass transition temperatures,T _g, for the IPNs, the lowerT _g corresponding to the PMMA phase and the higherT _g to the CF phase. However, the unusual increase inT _g of the CF from 128°C to 144°C suggests restrictions in the segmental motion of the CF phase brought about by mixing with another rigid polymer such as PMMA.     

Vinylester/epoxy-based thermosets of interpenetrating network structure: An atomic force microscopic study

The morphology of vinylester (VE) and epoxy (EP) resins, and their combination (VE/EP at a ratio 1/1) was studied by atomic force microscopy (AFM). AFM scans taken on the ion-etched surface of EP showed a featureless homogeneous structure. On the other hand, VE exhibited a two-phase microgel, whereas VE/EP a two-phase interpenetrating network (IPN) structure. A vinylester-urethane hybrid resin (VEUH) showed also the formation of an IPN-like morphology. It was concluded that cross-reactions between the hydroxyl functionality of VE and additional functional groups, such as isocyanate in VEUH or epoxy in VE/EP combinations, strongly favour the IPN formation. AFM scans revealed the compaction of the IPN structure of VE/EP upon post-curing which was associated with a prominent increase in the glass transition temperature (T _g) according to dynamic-mechanical thermal analysis.     

Embedded single carbon fibre to sense the thermomechanical behavior of an epoxy during the cure process

A new approach that uses a single carbon fibre for sensing the thermomechanical behavior of an epoxy during the cure cycle is presented. By recording and analyzing the electrical resistance and temperature history of a carbon fibre embedded inside an epoxy specimen during the cure cycle, the interaction between the carbon fibre and the surrounding polymer can be revealed. Compared with reported TMA and DMA results, this embedded carbon fibre sensor approach successfully detects the glass transition zone covering the final transition temperature, the main transition temperature, and the starting transition temperature that respectively have similar values as: (i) T_g corresponding to the abrupt change in CTE, (ii) T_g by storage modulus (E′) onset, and (iii) the upper temperature limit for the linear relationship between E′ and the temperature. The future applications for this sensor method are also discussed.     

Woven hybrid biocomposites: Dynamic mechanical and thermal properties

The dynamic mechanical and thermal analysis of oil palm empty fruit bunch (EFB)/woven jute fibre (J_w) reinforced epoxy hybrid composites were carried out. The storage modulus (E′) was found to decrease with temperature in all cases, and hybrid composites had showed better values of E′ at glass transition temperature (T_g) compared to EFB and epoxy. Loss modulus showed shifts in the T_g of the polymer matrix with the addition of fibre as reinforcing phase, which indicate that fibre plays an important role in case of T_g. The Tan δ peak height was minimum for jute composites and maximum for epoxy matrix. Complex modulus variations and phase behaviour of the hybrid composites was studied by Cole-Cole analysis. Thermal analysis result indicates an increase in thermal stability of EFB composite with the incorporation of woven jute fibres. Hybridization of EFB composite with J_w fibres enhanced the dynamic mechanical and thermal properties.     

Morphology and thermal properties of organic–inorganic hybrid material involving monofunctional-anhydride POSS and epoxy resin

Monofunctional-anhydride polyhedral oligomeric silsesquioxane (i-C₄H₉)₇Si₈O₁₂OSi(CH₃)₂(C₈H₉O₃) (AH-POSS) was synthesized and characterized by FTIR, NMR, element analysis. Then AH-POSS was incorporated into epoxy system either pre-reacted or non-reacted using hexahydrophthalic anhydride (HHPA) as curing agent. Pre-reacted system hybrid materials were obtained by two-step preparation. First, AH-POSS reacted with part of diglycidyl ether of bisphenol A (DGEBA) to form AH-POSS-epoxy precursor in DGEBA, then cured with HHPA. Non-reacted POSS/epoxy hybrid materials were prepared by directly mixing AH-POSS, HHPA and DGEBA together and cured afterwards. The GPC and FTIR spectra suggested successful bonding of AH-POSS and epoxy resin. Morphologies of hybrid materials were characterized by SEM and TEM. Non-reacted system led to a dispersion of spherical particles with sizes in the range of micrometers. For pre-reacted system, polymerization-induced phase separation took place with POSS content lower than 30 wt% and also some “vesicle” structure was formed after curing. A typical macro-phase separation happened with POSS content up to 40 wt% before and after curing. The glass transition temperatures (T_g's) and the storage modulus were measured by dynamic mechanical analysis (DMA). T_g's and modulus displayed irregularly decrease. The initial thermal decomposition temperatures (T_d's) characterized by TGA were also irregularly decreasing for both systems. However, they were higher than those of epoxy composites when using amine as the curing agent.     

Characterization of molecular orientation by differential scanning calorimetry

Differential scanning calorimetry was used to study relaxations occurring above the glass transition, T _g for a number of characterized and fabricated polystyrenes. Part 1 of this study examines the liquid-liquid relaxation temperature, T _LL; Part 2 the effects of injection moulding and biaxial orientation of polystyrene; and Part 3 deals with the effects of injection blow moulding.The rubbery plateau region was found to be bounded by the glass transition, T _g, and a weak transition or relaxation, T _LL, found above T _g as an endothermic slope change in the base line. In some cases, this transition could be enhanced as a step change when molten samples were quenched in liquid nitrogen.The effect of molecular weight on T _g and T _LL was similar, and had the relationships T _LL/T _g=1.07+0.03 forM _inw/M _n 1, and T _LL/T _g=1.15 ± 0.01 for ¯M _w/¯M _n > 1.